3 Answers
3

My working theory is that there are no meaningful differences in actions performed under the app as opposed to the DMG.

One could argue at a low level that the difference is that invoking the Install OS X Mountain Lion.app bundle loads app and UI frameworks that simply mounting a DMG does not, but that's not exactly the intent of your question I'm sure.

Any meaningful installer-level actions should be identical in either medium, since the majority of the resources are in the DMG, and the DMG is in the app. The app must mount the disk image in order to get at the necessary assets.

Actions described in the wiki for Munki

The postflight script performs the actions that the GUI
"Install Mac OS X Lion" or "Install OS X Mountain Lion" application
does when you choose to install OS X.

Those actions are:

Create a Mac OS X Install Data directory at the root of the target
volume.

Mount the InstallESD.dmg disk image.

Copy the kernelcache and boot.efi files from the disk image to the Mac OS X Install Data directory. (The kernelcache is copied to
the Recovery HD helper partition if the target volume is encrypted
with FileVault 2.)

Unmount (eject) the InstallESD.dmg disk image.

If the InstallLion.pkg is on the same volume as the target volume, create a hard link to the InstallESD.dmg disk image in
Mac OS X Install Data, otherwise copy the InstallESD.dmg disk
image to that directory.

Create a com.apple.Boot.plist file in the Mac OS X Install Data directory which tells the kernel how to mount
the disk image to use for booting. (This file is instead created on
the the Recovery HD helper partition if the target volume is encrypted
with FileVault 2.)

Create a minstallconfig.xml file, which tells the OS X Installer what to install and to which volume to install it. It also provides a
path to a MacOSXInstaller.choiceChanges file if one has been
included in the package.

Create an index.sproduct file and an OSInstallAttr.plist in the Mac OS X Install Data directory. These are also used by the OS X
Installer.

Set a variable in nvram that the OS X Installer uses to find the product install info after reboot.

Use the bless command to cause the Mac to boot from the kernel files copied to the Mac OS X Install Data directory.

Generalising for Mountain Lion, without Munki

For Ask Different, this answer requires more work.

Not definite at the moment, I assume that InstallLion.pkg is associated with a postflight script of Munki (not normally associated with Install OS X Mountain Lion.app).

Definite variations to what's quoted above:

not Mac OS X Install Data; the name of the directory is OS X Install Data

There is no difference whatsoever. The app is just there to create an Installer image on your disk. It restores InstallESD.dmg to the target partition and moves the data to a temporary folder. Then it asks you to reboot, and the installer continues by booting from the target partition.

Potentially misleading – to the best of my knowledge the app, running, does not add any partition. Partitioning may occur later in the routine (when something other than the app is running).
–
Graham PerrinAug 3 '12 at 8:58

I am sorry. I checked again, and I see it restores InstallESD.dmg to the Target Volume without erasing it. I was probably thinking about the Recovery HD when writing this answer.
–
duci9yAug 3 '12 at 13:40